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bastion host

(I) A strongly protected computer that is in a network protected
by a firewall (or is part of a firewall) and is the only host (or
one of only a few hosts) in the network that can be directly
accessed from networks on the other side of the firewall.

(C) Filtering routers in a firewall typically restrict traffic
from the outside network to reaching just one host, the bastion
host, which usually is part of the firewall. Since only this one
host can be directly attacked, only this one host needs to be very
strongly protected, so security can be maintained more easily and
less expensively. However, to allow legitimate internal and
external users to access application resources through the
firewall, higher layer protocols and services need to be relayed
and forwarded by the bastion host. Some services (e.g., DNS and
SMTP) have forwarding built in; other services (e.g., TELNET and
FTP) require a proxy server on the bastion host.